Subsea Heat Exchangers For Offshore Hydrocarbon Production Operations

ABSTRACT

A subsea heat exchanger is disclosed that includes a production fluid inlet, a production fluid outlet, and first and second heat exchanger units coupled to the inlet and outlet. Each heat exchanger unit includes an outer tubular member, an inner tubular member disposed within the outer tubular member, and an annulus radially disposed between the inner tubular member and the outer tubular member. In addition, each heat exchanger unit includes a bridging assembly coupled between the first heat exchanger unit and the second heat exchanger unit. The bridging assembly includes a connector including a throughbore in communication with the inner tubular member of the first heat exchanger unit and the inner tubular member of the second heat exchanger unit. In addition, the bridging assembly includes a tubular stab that fluidly couples the annulus of the first heat exchanger unit to the annulus of the second heat exchanger unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional patent applicationSer. No. 62/109,729 filed Jan. 30, 2015, and entitled: “Subsea HeatExchangers for Offshore Hydrocarbon Production Operations,” which ishereby incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

This disclosure relates generally to subsea hydrocarbon production. Moreparticularly, this disclosure relates to subsea heat exchangers for usewith offshore hydrocarbon production systems.

The temperatures of hydrocarbon bearing subterranean reservoirs canrange from very high (e.g., higher than 400° F.) to very low (e.g.,lower than −75° F.). The temperature of any given reservoir is typicallydictated by factors such as, for example, the composition of thematerials and fluids within the reservoir, the depth of the reservoir,and proximity to other geological features (e.g., hot spots, faults,etc.). During production from a reservoir, produced fluids havingextreme temperatures can push the operational limits of the productionequipment (e.g., manifolds, risers, piping, etc.), potentially resultingin damage to such equipment. These problems are exacerbated when theproduction operations are conducted offshore, where the wellhead andmuch of the production equipment is located on the sea floor, which maybe several thousand feet down from the sea surface.

BRIEF SUMMARY OF THE DISCLOSURE

Some embodiments disclosed herein are directed to a subsea heatexchanger. In an embodiment, the subsea heat exchanger includes aproduction fluid inlet and a production fluid outlet. In addition, thesubsea heat exchanger includes a first heat exchanger unit and a secondheat exchanger unit each coupled to the inlet and the outlet, whereineach heat exchanger unit has a central axis, a first end, and a secondend opposite the first end. Each heat exchanger unit includes an outertubular member extending axially from the first end to the second end ofthe heat exchanger unit, an inner tubular member disposed within theouter tubular member, wherein the inner tubular member extends axiallyfrom the first end to the second of the heat exchanger unit, and anannulus radially disposed between the inner tubular member and the outertubular member. In addition, each heat exchanger unit includes abridging assembly coupled to the second end of the first heat exchangerunit and the second end of the second heat exchanger unit. The bridgingassembly includes a connector having a central connector axis, a firstend coupled to the second end of the first heat exchanger unit, a secondend coupled to the second end of the second heat exchanger unit, and athroughbore in communication with the inner tubular member of the firstheat exchanger unit and the inner tubular member of the second heatexchanger unit. In addition, the bridging assembly includes a tubularstab having a central stab axis oriented parallel to and radially spacedfrom the connector axis, wherein the tubular stab fluidly couples theannulus of the first heat exchanger unit to the annulus of the secondheat exchanger unit.

Other embodiments disclosed herein are directed to an offshoreproduction system for producing hydrocarbon fluids from a subterraneanwell. In an embodiment, the system includes a production tree disposedat the sea floor, wherein the production tree includes a plurality ofvalves configured to control a flow of hydrocarbon fluids from thesubterranean well. In addition, the system includes a riser assemblyfluidly coupled to the production tree and configured to flow thehydrocarbon fluids to a vessel disposed at the sea surface. Further, thesystem includes a heat exchanger disposed on the sea floor. The heatexchanger includes an inlet configured to receive the hydrocarbon fluidsfrom the production tree and an outlet configured to supply thehydrocarbon fluids to the riser assembly. In addition, the heatexchanger includes a plurality of heat exchanger units coupled to theinlet and the outlet. Each heat exchanger unit has a central axis, afirst end, and a second end opposite the first end. In addition, eachheat exchanger unit includes an outer tubular member extending axiallyfrom the first end to the second end of the heat exchanger unit.Further, each heat exchanger unit includes an inner tubular memberdisposed within the outer tubular member, wherein the inner tubularmember extends axially from the first end to the second of the heatexchanger unit, and wherein the inner tubular member of each heatexchanger unit is in fluid communication with the inlet and the outlet.Still further, each heat exchanger unit includes an annulus radiallydisposed between the inner tubular member and the outer tubular member.Further, the heat exchanger includes a closed thermal processing loop influid communication with the annulus of each of the heat exchangerunits, wherein the thermal processing loop is configured to circulate athermal processing fluid through the annuli of the plurality of heatexchanger units.

Still other embodiments disclosed herein are directed to a method forcooling hydrocarbon fluids produced from an offshore subterranean well.In an embodiment, the method includes producing hydrocarbon fluids froma production tree disposed at the sea floor to an inlet, and flowing thehydrocarbon fluids from the inlet through an inner tubular member of afirst heat exchanger unit. In addition, the method includes flowing thehydrocarbon fluids through a connector of a bridging assembly into aninner tubular member of a second heat exchanger unit, and flowing athermal transfer fluid through an annulus of the first heat exchangerunit. Further, the method includes flowing the thermal transfer fluidthrough a tubular stab of the bridging assembly into an annulus of thesecond heat exchanger unit.

Embodiments described herein comprise a combination of features andadvantages intended to address various shortcomings associated withcertain prior devices, systems, and methods. The foregoing has outlinedrather broadly the features and technical advantages of the disclosedexemplary embodiments in order that the detailed description thatfollows may be better understood. The various characteristics describedabove, as well as other features, will be readily apparent to thoseskilled in the art upon reading the following detailed description, andby referring to the accompanying drawings. It should be appreciated bythose skilled in the art that the conception and the specificembodiments disclosed may be readily utilized as a basis for modifyingor designing other structures for carrying out the same purposes of theembodiments described herein. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the disclosed embodiments, reference willnow be made to the accompanying drawings in which:

FIG. 1 is a schematic view of an embodiment of an offshore productionsystem including a subsea heat exchanger in accordance with theprinciples disclosed herein;

FIG. 2 is a perspective view of the subsea heat exchanger of FIG. 1;

FIG. 3 is a cross-sectional side view of the subsea heat exchanger ofFIG. 1;

FIG. 4 is an enlarged cross-sectional view of an outer end of a heatexchanger unit within the subsea heat exchanger of FIG. 1;

FIG. 5 is a cross-sectional view of a bridging assembly extendingbetween two adjacent heat exchanger units within the subsea heatexchanger of FIG. 1;

FIG. 6 is an exploded, perspective view of a heat exchanger unit of thesubsea heat exchanger of FIG. 1;

FIG. 7 is an enlarged, perspective view of a baffle of the heatexchanger unit of FIG. 6;

FIG. 8 is a top schematic view of the subsea heat exchanger of FIG. 1illustrating the respective flow paths of the production fluid andthermal transfer fluid;

FIG. 9 is an enlarged perspective view of one end of the subsea heatexchanger of FIG. 1 illustrating the transfer pipes and tubesinterconnecting adjacent rows of heat exchanger units;

FIG. 10 is a perspective view of an embodiment of a subsea heatexchanger for use within the offshore production system of FIG. 1;

FIG. 11 is a perspective view of an embodiment of a subsea heatexchanger for use within the offshore production system of FIG. 1; and

FIG. 12 is a top schematic view of an embodiment of a subsea heatexchanger for use within the offshore production system of FIG. 1.

DETAILED DESCRIPTION

The following discussion is directed to various exemplary embodiments.However, one skilled in the art will understand that the examplesdisclosed herein have broad application, and that the discussion of anyembodiment is meant only to be exemplary of that embodiment, and notintended to suggest that the scope of the disclosure, including theclaims, is limited to that embodiment.

Certain terms are used throughout the following description and claimsto refer to particular features or components. As one skilled in the artwill appreciate, different persons may refer to the same feature orcomponent by different names. This document does not intend todistinguish between components or features that differ in name but notfunction. The drawing figures are not necessarily to scale. Certainfeatures and components herein may be shown exaggerated in scale or insomewhat schematic form and some details of conventional elements maynot be shown in interest of clarity and conciseness.

In the following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . .” Also, theterm “couple” or “couples” is intended to mean either an indirect ordirect connection. Thus, if a first device couples to a second device,that connection may be through a direct connection, or through anindirect connection via other devices, components, and connections. Inaddition, as used herein, the terms “axial” and “axially” generally meanalong or parallel to a central axis (e.g., central axis of a body or aport), while the terms “radial” and “radially” generally meanperpendicular to the central axis. For instance, an axial distancerefers to a distance measured along or parallel to the central axis, anda radial distance means a distance measured perpendicular to the centralaxis.

As previously described, production fluids having an extremely hot orcold temperature can stress production equipment (e.g., manifolds,risers, piping, etc.), potentially causing damage to such productionequipment. This can be particularly problematic in offshore productionoperations where much of the production hardware is disposed at the seafloor. Embodiments disclosed herein include heat exchangers and relatedequipment for use within a subsea hydrocarbon production system toeither raise the temperature of produced fluids having a very lowtemperature and lower the temperature of produced fluids having a veryhigh temperature to reduce the thermal stresses experienced byproduction equipment, thereby offering the potential to reduce thelikelihood of damage to such equipment and enhance the operatinglifetime of such equipment. In addition, the heat exchangers of at leastsome of the embodiments disclosed herein reduce the rate of corrosionfor subsea production equipment due, at least in part, to the reductionin the temperature of produced fluids. Further, the heat exchangers ofat least some of the embodiments disclosed herein reduce (or eliminate)the need for specialized equipment within the subsea production systemthat are designed and rated to receive and flow fluid of an extremetemperature (e.g., equipment including increased wall thickness,specialized materials, etc.).

Referring now to FIG. 1, an embodiment of an offshore hydrocarbonproduction system 10 in accordance with the principles described hereinis shown. Production system 10 facilitates the production of fluids froma wellbore 14 extending into a subterranean reservoir. In thisembodiment, production fluids comprise hydrocarbons, such as, forexample, liquid petroleum, natural gas, hydrocarbon condensate, andcombinations thereof. Production system 10 generally includes a subseaproduction tree 12 mounted to a wellhead 13, a subsea heat exchanger100, a manifold 16, a pipeline end termination (PLET) 18, and a riserassembly 20. Production tree 12, wellhead 14, heat exchanger 100,manifold 16, and PLET 18 are positioned along the sea floor 7. Inaddition, tree 12, heat exchanger 100, manifold 16, PLET 18, and riserassembly 20 are fluidly connected to one another with a plurality offluid conduits or jumper lines 17.

Wellhead 13 is disposed at the upper end of a cased wellbore 14, therebyfluidly coupling production tree 12 to wellbore 14. Production tree 12includes a plurality of valves 15 that control the flow of producedfluids from wellbore 14. Subsea heat exchanger 100 receives productionfluids from tree 12 and either raises or lowers the temperature of theproduced fluids, as appropriate, such that when the produced fluids exitexchanger 100 they are at an acceptable and/or predetermined temperaturewithin the safe operating temperature range of each piece of downstreamequipment (e.g., manifold 16, PLET 18, riser assembly 20, etc.).Manifold 16 receives production fluids from heat exchanger 100, and, incertain embodiments, also receives production fluids from otherwellbores (not shown). Thereafter, the production fluids are passed frommanifold 16, through PLET 18, and are routed to riser assembly 20. Inthis embodiment, riser assembly 20 includes a lower riser assembly 19disposed at or proximate the sea floor 7 and a marine riser 21 extendingvertically from lower riser assembly 19 to a surface vessel 22 disposedat the sea surface 9. Thus, during production operations, productionfluids are received by riser assembly 20 from PLET 18 at the lower riserassembly 19, and are subsequently routed to vessel 22 at the sea surface9 through riser 21. In general, riser 21 can be any suitable riser orconduit for routing production fluids from the sea floor to the seasurface, such as, for example, a free standing riser, a catenary riser,a top tensioned riser, or some combination thereof all while stillcomplying with the principles disclosed herein.

Referring now to FIG. 2, subsea heat exchanger 100 generally includes aproduction fluid inlet 101, a production fluid outlet 102, a pluralityof modular heat exchanger units 120 disposed between and fluidly coupledto the inlet 101 and outlet 102, and a thermal processing loop 200fluidly coupled to each of the units 120. In this embodiment, exchanger100 is configured to cool (i.e., remove thermal energy from) relativelyhot production fluids as they pass from inlet 101 to outlet 102;however, in other embodiments, the heat exchanger (e.g., exchanger 100)is configured to heat (i.e., transfer thermal energy to) relatively coldproduction fluids with. As will be described in more detail below,exchanger 100 may be referred to herein as a “modular” heat exchanger,since it is constructed from a plurality of modular heat exchanger units120 that can be added or removed as needed based on the specific heattransfer requirements and specifications of the associated productionsystem (e.g., system 10).

Referring now to FIGS. 2 and 3, in this embodiment, each heat exchangerunit 120 includes a central or longitudinal axis 125, a first or outerend 120 a, a second or inner end 120 b opposite outer end 120 a, anouter tubular member 130 extending axially between ends 120 a, 120 b,and an inner tubular member 140 concentrically disposed within outertubular member 130 and extending axially between ends 120 a, 120 b. Eachend 120 a, 120 b further includes a stiffening plate 122 that isconnected to and supports each outer tubular member 130 and innertubular member 140 within unit 120.

In this embodiment, heat exchanger units 120 are arranged into threeadjacent, parallel rows 110A, 110B, 110C. In particular, two units 120are disposed within each row 110A, 110B, 110C such that the units 120within each row 110A, 110B, 110C share a pair of common stiffening plate122 at each end 120 a, 120 b with the corresponding adjacent unitswithin the other rows 110A, 110B, 110C. One or more of the plates 122may include(s) a pad eye or other similar attachment device (not shown)configured to receive cables and rigging for lowering and/or raisingthat particular exchanger to and/or from the sea floor 7, respectively.Moreover, as best shown in FIG. 3, the units 120 within each row 110A,110B, 110C are arranged such that the inner end 120 b of each unit 120in a given row 110A, 110B, 110C is positioned axially adjacent the innerend 120 b of a corresponding unit 120 within that same row 110A, 110B,110C, and the central axes 125 of the units 120 within each respectiverow 110A, 110B, 110C are coaxially aligned (note: while only row 110A isshown in FIG. 3, it should be appreciated that each of the other rows110B, 110C are arranged the same).

Each outer tubular member 130 is concentrically disposed about axis 125and includes a first or outer end 130 a, a second or inner end 130 bopposite outer end 130 a, a radially outer cylindrical surface 130 cextending axially between ends 130 a, 130 b, and a radially innercylindrical surface 130 d extending axially between ends 130 a, 130 b.Each outer tubular member 130 extends axially between ends 120 a, 120 bof the corresponding heat exchanger unit 120, and thus, ends 120 a, 130a axially aligned and ends 120 b, 130 b are axially aligned. In thisembodiment, outer tubular member 130 is comprised of a metallic material(e.g., steel); however, it should be appreciated that a wide range ofmaterials may be used to construct member 130, such as, for example,carbon-fiber composite.

Outer ends 130 a are secured to the corresponding plate 122 and innerends 130 b are secured to the corresponding plate 122. In thisembodiment, ends 130 a, 130 b are rigidly secured to the correspondingplate 122 at ends 120 a, 120 b, respectively, by welding; however, ingeneral, any suitable method for securing two rigid components to oneanother may be used, such as, for example, bolts, rivets, adhesive, etc.In addition, each end 130 a, 130 b is coaxially aligned with an apertureor port 123 extending through plates 122 at ends 120 a, 120 b,respectively, such that when outer tubular member 130 is secured toplates 122 at ends 130 a, 130 b, an open passage is defined along axis125 between plates 122 by outer tubular member 130.

Each inner tubular member 140 is concentrically disposed within acorresponding outer tubular member 130 and includes a first or outer end140 a, a second or inner end 140 b opposite outer end 140 a, a radiallyouter surface 140 c extending axially between ends 140 a, 140 b, and aradially inner surface 140 d also extending axially between ends 140 a,140 b. Each inner tubular member 140 extends axially between ends 120 a,120 b of the corresponding heat exchanger unit 120, and thus, ends 120a, 140 a are axially aligned and ends 120 b, 140 b are axially aligned.Radially inner surface 140 d defines a throughbore 142 extending axiallybetween ends 140 a, 140 b. As will be described in more detail below,production fluids flow through throughbore 142 during productionoperations.

During assembly, inner tubular member 140 is inserted within thecorresponding outer tubular member 130 through port 123 of one of theplates 122 such that (a) inner tubular member 140 is concentricallydisposed within outer tubular member 130 as shown in FIG. 3; (b) outerend 140 a is proximate outer end 130 a; and (c) inner end 140 b isproximate inner end 130 b. In addition, once tubular member 140 is fullyinserted within tubular member 130 an annulus 132 is formed radiallybetween outer surface 140 c and inner surface 130 d that extends axiallybetween ends 120 a, 120 b. As will be described in more detail below,during production operations, a thermal transfer fluid flows throughannulus 132 to facilitate the transfer thermal energy (e.g., heat) awayfrom production fluids flowing within throughbore 142. Thermal transferfluid may comprise any suitable fluid for facilitating heat transfer(e.g., convective heat transfer) with another fluid or body. Forexample, thermal transfer fluid may comprise, for example, seawater,fresh water, corrosion inhibitors, ethylene glycol, propylene glycol,organic acid technology fluid (e.g., DEX-COOL® or ZEREX™), water-solubleoil, mineral oil or combinations thereof.

In this embodiment, the axial lengths of outer tubular members 130(e.g., the length measured axially between ends 130 a, 130 b) and innertubular members 140 (e.g., length measured axially between ends 140 a,140 b) are no larger than the standard length of commercially availablepipe, which in some cases is approximately 45 feet. Consequently, thetubulars for constructing tubular members 130, 140 may be purchased orotherwise acquired directly from the existing stock of a given supplierwithout the need to order custom length pipes. In addition, such alength also eliminates the need to weld or otherwise join multiplelengths of pipe to construct tubular members 130, 140, an activity whichadds time and costs to the manufacturing of exchanger 100. However, itshould be appreciated that such a length for shells 130 and tubes 140 isnot required and each may be disposed at any suitable length while stillcomplying with the principle disclosed herein.

Outer end 120 a of each heat exchanger unit 120 is fluidly connected toeither inlet 101, outlet 102, or outer end 120 a of another exchangerunit 120 in the immediately adjacent row (e.g., row 110A, 110B, 110C).Specifically, referring now to FIG. 4, in this embodiment, outer end 140a of each member 140 includes a flange 150 disposed within acorresponding port 123 on one of the plates 122 that mates with acorresponding flange connector (e.g., flange connector 60) on adjacentpiping in the manner described below in order to fluidly connect unit120 within exchanger 100. While only one end 120 a of a single exchangerunit 120 is shown in FIG. 4, it should be appreciated that each end 120a is generally configured to same. Flange 150 generally includes aradially outer annular surface 152 and an end face 154. Annular surface152 includes a pair of axially adjacent seal assemblies 151. Each sealassembly 151 includes an annular recess 153 extending radially inwardfrom annular surface 152 and an annular sealing member 155 (e.g., anS-Seal, lip seal, T-seal, O-ring, etc.) disposed within recess 153. Wheninner tubular member 140 is installed within outer tubular member 130,annular surface 152 slidingly engages both port 123 and radially innersurface 130 d and each sealing member 155 is radially compressed betweensurface 130 d and the corresponding recess 153. As a result, duringproduction operations, fluid flow between surfaces 130 d, 153 isrestricted and/or prevented by the annular sealing assemblies 151. Face154 includes a generally planar engagement surface 154 a that extendsannularly about throughbore 142 and includes an annular recess 156 thatextends axially inward from engagement surface 154 a.

As is shown in FIG. 4, flange 150 at outer end 140 a mates and engages acorresponding flange 160 disposed on an end of an exterior pipe (e.g.,inlet 101, outlet 102, etc.). In particular, engagement surface 154 a onface 154 mates and engages a corresponding engagement surface 164 a on aface of mating flange 160. Engagement surface 164 a on mating flange 160also includes an annular recess 166 that extends radially inward fromsurface 164 a such that when flanges 150, 160, are mated (e.g., bolted)with one another as shown, recesses 156, 166 are generally aligned. Anannular sealing member 158 (e.g., metallic or elastomeric gasket) isdisposed within the aligned recesses 156, 166 and is compressedtherebetween such that fluid flow between the surfaces 154 a, 164 a isrestricted and/or prevented by the compressed sealing member 158.Additional sealing member(s) (e.g., elastomeric sealing ring(s)) may bedisposed about the outer periphery of sealing member 158 to preventseawater from contacting and thus compromising the integrity of member158 and/or serve as a secondary barrier to prevent fluid flow betweensurfaces 154 a, 164 a.

Referring still to FIG. 4, the radially outer portion of each flange 150includes a plurality of uniformly circumferentially-spaced apertures orports 159 extending axially therethrough. Each port 159 receives the endof a transfer tubing member 222 which, as will be described in moredetail below, delivers thermal transfer fluid into and out of annulus132 to facilitate thermal energy transfer with production fluid flowingwithin throughbore 142 during production operations. Fluid flow betweeneach tubing member 222 and the corresponding flange 150 is restrictedand/or prevented by an annular sealing assembly (not shown) disposedradially therebetween. In addition, as is best shown in FIG. 9, thediameter of each flange 160 is smaller than the diameter of thecorresponding flange 150 such that each tubing member 222 is positionedradially outside connector 160 as it extends axially into thecorresponding port 159.

Referring now to FIGS. 3 and 5, inner ends 120 b of the coaxiallyaligned heat exchanger units 120 within each row 110A, 110B, 110C areconnected to one another with a connection or bridging assembly 170. Onebridging assembly 170 will now be described it being understood theother bridging assemblies 170 are the same. As best shown in FIG. 5, inthis embodiment, bridging assembly 170 includes a central connector 172extending between opposed inner ends 140 b and a plurality of uniformlycircumferentially-spaced tubular members or stabs 176 disposed aboutconnector 172. Central connector 172 is an elongate tubular memberincluding a central axis 173 that is aligned with the axes 125 of eachof the heat exchanger units 120 within row 110A, a first end 172 a, asecond end 172 b opposite first end 172 a, a radially outer surface 172c extending axially between ends 172 a, 172 b, and a radially innersurface 172 d extending axially between ends 172 a, 172 b. Inner surface172 d defines a throughbore 174 extending axially between ends 172 a,172 b. Each end 172 a, 172 b includes an annular flange 160 aspreviously described. In addition, inner ends 140 b of tubes 140 eachinclude a flange 150 that is configured the same as flange 150 on outerends 140 a. Thus, as is shown in FIG. 5, annular surface 152 of flange150 at inner end 140 b slidingly and sealing engages with the radiallyinner surface 130 d and aperture 123 of the corresponding shell 130 andplate 122, respectively, in the same manner as described above for outerend 120 a.

Each end 172 a, 172 b is connected to the inner end 140 b of one of thetubular members 140 via engagement of mating flanges 150, 160.Specifically, as shown in FIG. 5, flange 160 at first end 172 a ofcentral connector 172 mates with and engages flange 150 of inner end 140b of inner tubular member 140 disposed within one of the units 120within row 110A (i.e., the unit 120 on the left side of FIG. 5), whileflange 160 at second end 172 b mates with and engages flange 150 oninner end 140 b of inner tubular member 140 disposed within the otherunit 120 within row 110A (i.e., the unit 120 on the right side of FIG.5). As a result, throughbore 174 in central connector 172 is coaxiallyaligned with both throughbores 142 in tubes 140 within row 110A, therebyforming a continuous production fluid flow path 112A extending axiallybetween units 120 of row 110A. As is shown schematically in FIG. 8,production fluid flow paths 112B, 112C extend axially through rows 110B,110C, respectively, and are configured the same as fluid flow path 112Adescribed above.

Referring again to FIGS. 3 and 5, each stab 176 of bridging assembly 170is an elongate tubular member including a first end 176 a, a second end176 b opposite first end 176 a, a radially outer cylindrical surface 176c extending axially between ends 176 a, 176 b, and a radially innercylindrical surface 176 d extending axially between ends 176 a, 176 b.Inner surface 176 d defines a throughbore 178 extending axially throughstab 176. As best shown in FIG. 5, each port 159 of flange 150 on oneinner end 140 b is circumferentially aligned with one port 159 of theopposed flange 150 of the other inner end 140 b. One stab 176 extendsaxially through each pair of aligned ports 159—first end 176 a beingreceived within one port 159 on one of the units 120 within row 110A andsecond end 176 b being received within the aligned port 159 on the otherunit 120 within row 110A. Radially outer surface 176 c of each stab 176is allowed to slidingly engage at least one of the corresponding ports159 such that units 120 within row 110A may move axially relative tostabs 176, thereby allowing bridging assembly 170 to accommodate thethermal expansion of units 120 during production operations. In thisembodiment, one end (e.g., end 176 a) is fixed within the correspondingport 159 such that this “fixed” end does not move axially relative tothat port 159, whereas the other end (e.g., end 176 b) is movablydisposed within its corresponding port 159 such that this “free” end canmove axially relative to its port 159. In general, any suitable methodfor fixing an end 176 a, 176 b within the corresponding port 159 may beused while still complying with the principles disclosed herein, suchas, for example, welding, a retention sleeve or locking ring disposedwithin one of the ports 159, etc. In addition, one or more sealingassemblies (not shown) are included between radially outer surface 176 cof stabs and ports 159 to restrict fluid flow therebetween duringoperations. For example, in some embodiments, an annular seal glandextends radially inward from radially outer surface 176 c and houses anannular sealing member (e.g., an O-ring, sealing ring, etc.) thatfurther engages with port 159; however, it should be appreciated thatany other suitable sealing assembly may be used while still complyingwith the principles disclosed herein. Together the annuli 132 andthroughbores 178 of stabs 176 define a continuous thermal fluid flowpath 113A through adjacent units 120 within row 110A. As is shownschematically in FIG. 8, thermal fluid flow paths 113B, 113C extendaxially through rows 110B, 110C, respectively, and are configured thesame as thermal fluid flow path 113A described above.

Referring again to FIG. 3, each heat exchanger unit 120 is supported onsea floor 7 with a plurality of support members 126. More specifically,each outer end 120 a is supported by a pair of outer support members 126a, and each inner end 120 b is supported by an inner support member 126b. Each support member 126 a, 126 b includes a base or foot 127 disposedalong the sea floor 7 and a plurality of columns 128 extendingvertically upward from foot 127. In this embodiment, inner supportmember 126 b includes two rows 129′, 129″ of columns 128 and outersupport members 126 a each include only a single row of columns 128.Each support column 128 on both inner and outer support members 126 a,126 b, respectively, includes a saddle 124 that receives one of thestiffening plates 122 on ends 120 a, 120 b. Specifically, saddles 124 incolumns 128 on outer support members 126 a receive the stiffening plates122 disposed at outer ends 120 a of units 120 within rows 110A, 110B,110C, while the saddles 124 on columns 128 on inner support member 126 breceive stiffening plates 122 disposed at inner ends 120 b of units 120in rows 110A, 110B, 110C. In addition, as is best shown in FIGS. 4 and5, each saddle 124 has an width W₁₂₄ measured axially relative to axis125 that is greater than the axial thickness T₁₂₂ of each plate 122.Each plate 122 slidingly engages the corresponding saddle 124, and thus,during production operations, plates 122 are free to slide axiallywithin saddles 124, such as, for example, to accommodate thermalexpansion or contraction of units 120. In some embodiments, engagedsurfaces of plates 122 and saddles 124 are finished or coated with asuitable surface treatment in order to reduce friction therebetweenduring operations. In still other embodiments, additional frictionreducing mechanisms may be employed between plates 122 and saddles 124,such as, for example, rollers, bearings, etc.

Referring now to FIGS. 3, 6, and 7, each unit 120 includes a pluralityof axially-spaced generally D-shaped baffles 180 disposed within annulus132 and fixably attached to the corresponding inner tubular member 140between ends 140 a, 140 b. As will be described in more detail below,baffles 180 function primarily to direct the flow of heat transfer fluidwithin annulus 132 during production operations. In addition, in atleast some embodiments, baffles 180 also form fin-like appendages alongeach inner tubular member 140 that effectively increases the surfacearea of outer surface 140 c, thereby enhancing thermal energy transferbetween thermal transfer fluids in annulus 132 and production fluidswithin throughbore 142 during operations. In this embodiment, each unit120 includes a total of five uniformly axially spaced baffles 180mounted to each inner tubular member 140; however, it should beappreciated that the number, arrangement, and axial spacing of baffles180 may be greatly varied while still complying with the principlesdisclosed herein.

Referring now to FIG. 7, one baffle 180 will now be described it beingunderstood each baffle 180 is the same. In this embodiment, each baffle180 is constructed from a pair of rigid plate members that are joined toone another. In particular, each baffle 180 includes a first or mainplate member 182 and a second or locking plate member 190 secured tomain plate member 182 with a pair of attachment assemblies 188. Mainplate member 182 is generally C-shaped and includes a first planar side180 a, a second planar side 180 b facing in the opposite direction asfirst side 180 a, a planar end surface 181 extending axially betweensides 180 a, 180 b, and a generally cylindrical surface 183 extendingaxially between sides 180 a, 180 b and circumferentially between theends of planar end surface 181. A U-shaped recess or notch 184 extendsradially inward from end surface 181. Recess 184 is defined by a pair ofradially oriented planar surfaces 185 and a curved surface 186 extendingcircumferentially about axis 125 between planar surfaces 185. As will bedescribed in more detail below, inner tubular member 140 is receivedwithin notch 184, and thus, the radius of curvature of surface 186 isthe same as the radius of curvature of radially outer surface 140 c ofinner tubular member 140. Similarly, as will also be described in moredetail below, curved outer surface 183 engages the inner surface 130 dof outer tubular member 130 when baffle 180 is installed within unit120, and thus, the radius of curvature of outer surface 130 is the sameas the radius of curvature of inner surface 130 d of outer tubularmember 130. A pair of apertures or through holes 187 extend axiallybetween sides 180 a, 180 b on opposite sides of notch 184. As will bedescribed in more detail below, each aperture 187 receives a bolt 189 tosecure main plate member 182 to locking plate member 190.

Referring still to FIG. 7, locking plate member 190 includes a firstplanar side 190 a, a second planar side 190 b facing away from firstside 190 a, a first planar end surface 191 extending axially betweensides 190 a, 190 b, a second planar end surface 192 extending axiallybetween sides 190 a, 190 b, and a pair of radially outer generallycylindrical surfaces 193 extending circumferentially between planar endsurfaces 191, 192. A cylindrical recess or notch 194 extends radiallyinward from second planar end surface 192 and is defined by a curvedsurface 195. As will be described in more detail below, inner tubularmember 140 is received within notch 194, and thus, the radius ofcurvature of surface 195 is the same as the radius of curvature ofradially outer surface 140 c of inner tubular member 140. A pair ofapertures or throughbore holes 196 extend axially between sides 190 a,190 b on opposite sides of notch 194. When baffle 180 is installedwithin the corresponding heat exchanger unit 120, curved surfaces 193generally align with the curved surface 183, and thus, like surface 183,each of the surfaces 193 also engage with the radially inner surface 130d of outer tubular member 130. Accordingly, like surface 183, the radiusof curvature of surface 193 is the same as the radius of curvature ofradially inner surface 130 d.

Each attachment assembly 188 includes a threaded rod or bolt 189 and apair of threaded nuts 197. To assemble baffle 180, main plate member 182is disposed along inner tubular member 140 at a desired location suchthat inner tubular member 140 is seated within notch 184 and radiallyouter surface 140 c is engaged with curved surface 186. Thereafter,first side 190 a of locking plate body 190 is engaged with second side180 b of main plate member 182 such that the apertures 187 in member 182are substantially aligned with the apertures 196 in member 190 andradially outer surface 140 c of inner tubular member 140 is engaged withcurved surface 195 within notch 194. Each of the bolts 189 of assemblies188 is then inserted within one pair of the aligned apertures 187, 196and nuts 197 are threadably engaged to bolts 189 along each of the firstside 180 a of member 182 and second side 190 b of member 190, therebyurging second side 180 b of member 182 into engagement with first side190 a of member 190, and securing baffle 180 to inner tubular member 140through a friction fit. It should be appreciated that baffles 180, onceassembled, can be further secured to radially outer surface 140 c oftubular member 140 by welding, adhesive, or some other suitable method,while still complying with the principles disclosed herein.

Referring back now to FIGS. 3 and 6, baffles 180 are disposed alonginner tubular member 140 in an alternating fashion with each baffle 180being rotated approximately 180° relative to the each immediatelyaxially adjacent baffle 180 with respect to axis 125. As a result, whenthermal transfer fluid is routed through annulus 132 between ends 120 a,120 b of each unit 120, it is forced to flow generally sinusoidallyaround baffles 180. Without being limited to this or any other theory,such a sinusoidal like flow pattern promotes turbulence within thethermal transfer fluid, which further enhances the transfer of thermalenergy between production fluids flowing within throughbore 142 of innertubular member 140 and the thermal transfer fluid flowing within annulus132. To prevent fluid flow between outer curved surfaces 183, 193 ofmembers 182, 190, respectively, radially outer curved surfaces 183, 193of baffles 180 (e.g. see FIG. 7) sealingly engage with radially innersurface 130 d. As a result, thermal transfer fluid flowing thoughannulus 132 is forced to flow around baffles 180 proximate planar endsurfaces 181, 191, thereby promoting the sinusoidal like flow patterndescribed above. In general, surfaces 183 and/or 193 may sealinglyengage radially inner surface 130 d of shell through any suitable methodwhile still complying with the principles disclosed herein. For example,surfaces 183 and/or 193 may engage surface 130 d with an integral seal,a pre-formed seal, a molded seal, etc. Without being limited to this orany other theory, in embodiments where an elastomeric type seal isutilized between surfaces 183 and/or 193 and 130 d, vibrational energyis at least partially absorbed by the elastomer forming the seal duringoperation, which thus reduces fatigue wear on inner tubular member 140.Further, in some embodiments, surfaces 183 and/or 193 are welded orotherwise adhered to radially inner surface 130 d.

Referring now to FIGS. 2, 8, and 9, during production operations,production fluid flows between inlet 101 and outlet 102 of exchanger100. During this process, thermal transfer fluid is also routed alongthermal processing loop 200 in order to facilitate the transfer ofthermal energy with production fluids. Specifically, in this embodiment,the flowing of thermal transfer fluid along thermal processing loop 200causes cooling of production fluids such that the temperature ofproduction fluid at outlet 102 is less than the temperature ofproduction fluid at the inlet 101. Thermal processing loop 200 generallyincludes a pumping unit 210, an outlet line 220 extending from pumpingunit 210 to heat exchanger units 120, a recirculation line 230 extendingfrom units 120, a thermal expansion section 240, and a cooling unit orradiator 250.

As best shown in FIG. 2, pumping unit 210 includes one or more pumpsthat are disposed within a support frame 212 resting on a mud mat 213.In this embodiment, two pumps 211 a, 211 b are disposed within pumpingunit 210. Pumps 211 a, 211 b are variable speed pumps configured suchthat their speeds may be adjusted by, for example, a controller unit. Asthe speed of the pumps 211 a, 211 b within unit 210 is increased, theflow rate and/or discharge pressure also increases. Similarly, as thespeed of the pumps 211 a, 211 b within unit 210 is decreased, the flowrate and/or discharge pressure also decreases. During installation,after the other components of exchanger 100 are installed on the seafloor 7, the pumping unit 210 is lowered, such as, for example, bysuspension from a pad eye 214 (e.g., See FIG. 2) attached to unit 210,until it is seated within support frame 212. Thereafter, all associatedpiping (e.g., outlet line 220) is fluidly connected to pumping unit 210by any suitable method, such as, for example, with a remotely operatedvehicle (ROV). If at some point, it becomes desirable to replace orrepair the pumping unit 210, the steps for installing the pumping unit210 are simply performed in reverse order. Specifically, all piping(e.g., outlet line 220) is disconnected and removed and unit 210 islifted out of frame 212 to the surface by, for example, a cableconnected to pad eye 214. Thus, pumping unit 210 may be referred toherein as a “retrievable” pumping unit 210 or a “separately retrievable”pumping unit 210, as its installation and removal is independent fromthe installation and removal of the other components of exchanger100—meaning, for example, that pumping unit 210 may be removed withoutrequiring the removal of any other components within exchanger 100.

Thermal expansion section 240 is included within loop 200 to accommodateany thermal expansion that might occur within any of the associatedlines (e.g., recirculation line 230) during operations. As shown inFIGS. 2 and 8, expansion section 240 is disposed along recirculationline 230 upstream of radiator 250. Referring specifically to FIG. 2, inthis embodiment expansion section 240 includes a U-shaped bend 242 thatdefines a space 244 sized to accommodate any axial increase in thelength of recirculation line 230 due to thermal expansion.

Referring again to FIGS. 2 and 8, radiator 250 is disposed along loop200 upstream of pumping unit 210 and downstream of expansion section240. In this embodiment, radiator 250 is configured to remove thermalenergy (e.g., heat) collected by thermal transfer fluid being circulatedthrough heat exchanger units 120. Radiator 250 includes a series of pipecurves or bends 252 connected by a plurality of straight sections ofpipe 254. Each of the bends 252 and straight sections 254 are exposed tothe surrounding ocean environment. Specifically, without being limitedto this or any other theory, each of the bends 252 and pipes 254 addsconsiderable length to the fluid path that spent thermal transfer fluidmust traverse to return to pumping unit 210. This increased lengthgreatly increases the surface area of pipe that is exposed to therelatively cool ocean environment, and thus promotes a greater transferof thermal energy between the thermal transfer fluids flowing throughradiator 250 and the surrounding sea water during operations.

Referring still to FIGS. 2, 8, and 9, during production operations, bothproduction and fluid and thermal transfer fluid are routed throughexchanger 100 to facilitate the transfer of thermal energy therebetween.For convenience, in FIG. 8 the flow path of production fluid withinexchanger 100 is shown by solid line arrows 105, while the flow ofthermal processing fluid through loop 200 is shown by broken line arrows205. Specifically, during operations, production fluid enters exchanger100 at inlet 101 and is then routed successively through each of therows 110A, 110B, 110C toward outlet 102. As production fluid travelsthrough rows 110A, 110B, 110C, it flows successively through theproduction fluids flow paths 112A, 112B, 112C within each row 110A,110B, 110C, respectively. Upon exiting one of the production fluid flowpaths 112A, 112B, 112C, production fluids are then routed through atransfer pipe to the next successive fluid flow path (e.g., path 112B,112C in rows 110B, 110C, respectively). For example, upon exiting fluidflow path 112A within first row 110A, production fluids are routedthrough a first transfer pipe 104 to fluid flow path 112B within secondrow 110B, and upon exiting fluid flow path 112B, a second transfer pipe106 routes the production fluids to fluid flow path 112C within thirdrow 110C. Each transfer pipe 104, 106 includes a pair of flangeconnectors 160 that mate with the corresponding flange connectors 150 onthe respective units 120 within rows 110A, 110B, 110C, in the samemanner as previously described above (e.g., see FIG. 4 and theassociated description) (see also FIG. 9). Similarly, both inlet 101 andoutlet 102 include flange connectors 160 that mate with thecorresponding flange connectors 150 on tubes 140 within rows 110A, 110Cin the same manner as previously described above (e.g., see FIG. 4 andthe associated description).

Referring still to FIGS. 2, 8, and 9, as production fluid is routedbetween inlet 101 and outlet 102 as described above, thermal transferfluid is routed along thermal processing loop 200 to facilitate coolingof production fluids. Specifically, pressurized thermal transfer fluidis discharged by pumping unit 210 into outlet line 220 where it isrouted into a manifold 221 and split into a plurality of transfer tubes222, the ends of which are installed within the ports 159 on flange 150at outer end 140 a of inner tubular member 140 within one of the heatexchanger units 120 of first row 110A as previously described (e.g., SeeFIG. 4 and the associated description). Thus, tubes 222 provide fluidcommunication with the annulus 132 in one of the units 120 within firstrow 110A. Thereafter, thermal transfer fluid is routed successivelythrough each of the thermal transfer fluid flow paths 113A, 113B, 113Cwithin rows 110A, 110B, 110C, respectively, toward recirculation line230. Specifically, upon exiting the fluid flow path 113A, 113B, 113Cwithin a given row 110A, 110B, 110C, respectively, thermal transferfluid is then routed through a plurality of transfer tubes 222 to thenext successive row (e.g., row 110B, 110C). In this embodiment, uponexiting fluid flow path 113A within first row 110A, a first set oftransfer tubes 222′ routes the thermal transfer fluid to the flow path113B within second row 110B, and upon exiting flow path 113B, a secondset of transfer tubes 222″ routes the thermal transfer fluid to fluidflow path 113C within third row 110C. After exiting fluid flow path113C, the now spent thermal transfer fluid is routed through a third setof transfer tubes 222′″ into a manifold 223 (which is substantially thesame as the manifold 221), which further directs the fluid intorecirculation line 230. Thus, in this embodiment thermal transfer fluidsare routed through heat exchanger units 120 in a parallel flowarrangement with production fluids (i.e., where thermal transfer fluidsflow in the same general axial direction along paths 113A, 113B, 113C asproduction fluid along flow paths, 112A, 112B, 112C, respectively,during operations). However, it should be appreciated that in otherembodiments, thermal transfer fluids are routed in a counter flowarrangement with production fluids (i.e., where thermal transfer fluidsflow in a generally opposite axial direction along paths 113A, 113B,113C as production fluid along flow paths 112A, 112B, 112C,respectively, during operations).

In this embodiment, thermal transfer fluid within flow paths 113A, 113B,113C has a temperature less than the temperature of the productionfluids within flow paths 112A, 112B, 112C, respectively (i.e.,production fluids are relatively hot in this case). Thus, as thermaltransfer fluid flows through the annuli 132 of heat exchanger units 120and the relatively warm production fluids flow through tubes 140,thermal energy is transferred from the production fluids through tubes140 to the thermal transfer fluid. Thus, as production fluid flowswithin throughbores 142 of tubes 140 in rows 110A, 110B, 110C itgradually decreases in temperature such that it is at a minimumtemperature when it reaches outlet 102. Conversely, as thermal transferfluid flows through the units 120 in rows 110A, 110B, 110C, it graduallyincreases in temperature such that it is at a maximum temperature whenit reaches recirculation line 230. As a result, upon enteringrecirculation line 230, the spent, warm thermal transfer fluid flowsthrough expansion section 240 into radiator 250 where the thermaltransfer fluid is cooled in the manner described above. Thereafter, thenow cooled thermal transfer fluid is once again routed through pumpingunit 210, thereby repeating the process described above. Thus, thethermal transfer fluid is continuously re-circulated through exchanger100 and loop 200 throughout heat transfer operations.

Referring now to FIG. 8, during the above described thermal processingoperations, a plurality of parameter measurements (e.g., temperature,pressure, flow rate, viscosity, etc.) can be taken at various pointswithin exchanger 100 and fed (e.g., wirelessly, through cables, etc.) toa controller unit (not shown) that may be disposed subsea, on vessel 20,or at some other location, such that performance of exchanger 100 can bemonitored and adjusted as necessary based on the performance andspecifications of the corresponding production system (e.g., system 10).In this embodiment, a plurality of measurement assemblies 260 aredisposed throughout exchanger 100, namely at inlet 101, outlet 102,recirculation line 230, and outlet line 220. Each measurement assembly260 measures one or more of the temperature, pressure, and flow rate ofthe fluid flowing through the corresponding tubular (e.g., productionfluid, thermal transfer fluid), and then communicates the measurement(s)to the controller unit. In this embodiment, the controller unit isdisposed on vessel 20. Thereafter, either personnel, softwareapplications, or some combination thereof, analyze the measurements fromassemblies 260 and determine what, if any, adjustments need to be madeby the controller unit to the operating parameters of exchanger 100(e.g., the pump speed) in order to achieve a predetermined, desiredperformance. Specifically, in some embodiments, the controller unitadjusts the flow rate of thermal transfer fluid based on themeasurements made by assemblies 260 (e.g., temperature) to therebyadjust and control the rate of heat transfer (e.g., convective) betweenproduction fluids and thermal transfer fluids. In this embodiment, theflow rate of thermal transfer fluid through exchanger 100 is adjusted byadjusting the speed of pumps 211 a, 211 b within pumping unit 210. Inaddition, in at least some embodiments, the controller unit may eitheradditionally or alternatively adjust the flow rate of production fluidsflowing to inlet 101 by, for example, actuating a choke valve that isdisposed upstream of exchanger 100 (e.g., on tree 12). Further, in atleast some embodiments, one or more heating elements may be installed atcertain locations within exchanger 100 (e.g., along line 220 betweenunit 210 and heat exchanger units 120) that can be utilized to alter(e.g., increase) the temperature of the thermal transfer fluid flowingtherethrough and thus further adjust the performance of exchanger 100.Although sensors assemblies 260 are only shown at inlet 101, outlet 102,line 230, and line 220 in this embodiment, in general, sensor assemblies260 can be disposed at various other locations within exchanger 100(e.g., within one or more of the heat exchanger units 120) either inaddition to or in lieu of the locations shown in FIG. 8.

Referring now to FIGS. 2, 8, 10, and 11, as previously described,exchanger 100 may be referred to herein as a “modular” heat exchangersince the construction of exchanger 100 is ultimately based on aplurality of interconnected heat exchanger units 120. Thus, depending onthe needs and specifications of the given production system (e.g.,system 10), the number and arrangement of heat exchanger units 120 maybe modified such that the heat transfer performance delivered byexchanger 100 is optimized in light of those needs and specifications.

In particular, to increase the amount of thermal transfer withinexchanger 100 (i.e., to increase the amount of temperature change forproduction fluids between inlet 101 and outlet 102), the number of heatexchanger units 120 may simply be increased. This increase in units 120within exchanger 100 can be accomplished in a number of different ways,all while still complying with the principle disclosed herein, andessentially works to increase the ultimate length of the flow path forproduction fluids between inlet 101 and outlet 102. For example, in someembodiments, plates 122 may be enlarged (i.e., extended) in the radialdirection relative to the direction of axes 125 and one or moreadditional rows (e.g., 110A, 110B, 110C) of units 120 may be added. Asanother example, in some embodiments, each row 110A, 110B, 110C mayinclude one or more additional units 120 (other than simply two in theembodiment of FIGS. 2 and 8). In these embodiments, it should beappreciated that each of the units 120 of a given row 110A, 110B, 110Cwill be interconnected with bridging assemblies 170 as previouslydescribed above.

Conversely, to decrease the amount of thermal transfer within exchanger100 (i.e., to decrease the amount of temperature change for productionfluids between inlet 101 and outlet 102), the number of heat exchangerunits 120 may simply be decreased. This decrease in units 120 withinexchanger 100 can be accomplished in a number of different ways, allwhile still complying with the principle disclosed herein, andessentially works to decrease the ultimate length of the flow path forproduction fluids between inlet 101 and outlet 102. For example, as isshown in FIG. 10, in some embodiments, plates 122 may be shortened inthe radial direction relative to the direction of axes 125 and one ormore rows (e.g., rows 110A, 110B, 110C) of units 120 may be removed. Asanother example, in some embodiments, one or more heat exchanger units120 may be removed from each row 110A, 110B, 110C. Specifically, asshown in FIG. 11, in some embodiments, each row 110A, 110B, 110C onlyincludes a single unit 120, and thus, no bridging members 170 areincluded as no two units 120 are axially aligned along axes 125 in themanner described above.

Although subsea heat exchanger 100 is described above for use intransferring thermal energy from production fluids to the thermaltransfer fluid to cool the production fluids, embodiments describedherein can also be used to transfer thermal energy from the thermaltransfer fluid to the production fluids to warm the production fluids.For example, referring now to FIG. 12, a subsea heat exchanger 300 foruse within production system 10 is shown. In this embodiment, exchanger300 transfers thermal energy to production fluids as they are routedbetween inlet 101 and outlet 102.

Exchanger 300 includes many of the same components and features asexchanger 100 previously described, and thus, like numerals are used todescribe shared components between exchangers 100, 300 and thedescription below will focus only on the differences of exchanger 300relative to exchanger 100. As shown in FIG. 12, in addition to featuresof exchanger 100, exchanger 300 generally includes a pumping unit 310 inplace of pumping unit 210 previously described, and a warmingrecirculation line 330.

Pumping unit 310 is the same as pumping unit 210 except that pumpingunit 310 additionally includes one or more warming devices 311 disposedtherein that are configured to warm or heat the thermal transfer fluidthat is discharged thereby. In particular, in some embodiments, warmingdevices 311 within pumping unit 310 include one or more energy elements(e.g., resistive coils) that are electrically powered to generate heatthat is transfer to thermal transfer fluid during operations. However,it should be appreciated that any suitable heating elements forincreasing the temperature of the thermal transfer fluid flowing throughpumping unit 310 may be used while still complying with the principlesdisclosed herein. It should also be appreciated that in someembodiments, similar heating elements are disposed throughout theexchanger 300 at various locations (either in lieu of or in addition tothe pumping unit 310). In some of these embodiments, heating elementsare disposed within a separate retrievable unit, and in still others ofthese embodiments, heating elements are permanently installed at variouslocations within exchanger 300.

Warming recirculation line 330 extends from a valve assembly 320disposed along recirculation line 230 to pumping assembly 310. Asopposed to line 230, recirculation line 330 does not include a radiator250 or similar component configured to cool the fluids flowingtherethrough (e.g., along arrows 205). Rather, the intent in flowingthermal transfer fluid through line 330 is to maintain a giventemperature of the spent fluid after it exits heat exchanger units 120such that heating operations carried out in pumping unit 310 aspreviously described (e.g., with one or more energy elements) areenhanced. As a result, in this embodiment, all portions of recirculationline 330 are covered with thermal insulation such that heat loss fromthe thermal transfer fluid to the ocean environment is minimized. Inthis embodiment, valve assembly 320 is actuatable between at least threedifferent positions: (1) a first position in which thermal transferfluids are allowed to flow freely between fluid flow path 113C in row110C and recirculation line 230 but are restricted from flowing into andthrough line 330; (2) a second position in which thermal fluids areallowed to flow freely between fluid flow path 113C in row 110C andrecirculation line 330 but are restricted from flowing into and throughline 230; and (3) a third position in which thermal fluids arerestricted from flowing into and through both lines 230, 330. Ingeneral, valve assembly 320 can be actuated by any suitable method, suchas, for example, manual actuation by ROV or other interaction device,automatic actuation by a remote controller unit, etc. In addition, in atleast some embodiments, recirculation line 330 may include one or morethermal expansion sections 240 being the same as that included on line230 and previously described above.

During operations, production fluid flows along fluid flow paths 112A,112B, 112C within rows 110A, 110B, 110C, respectively, of heat exchangerunits 120 as previously described. Similarly, thermal transfer fluidflows along fluid flow paths 113A, 113B, 113C within rows 110A, 110B,110C, respectively, of heat exchanger units 120 as previously described.However, in this embodiment, prior to injection into heat exchangerunits 120 and flowing along paths 113A, 113B, 113C, thermal transferfluids are warmed/heated by the heating devices 311 disposed withinpumping unit 310, preferably to a temperature that is greater than thetemperature of the production fluids entering exchanger 100 at inlet101. Due to the differences in temperature, as the production fluids andthermal transfer fluids flow along fluid flow paths 112A, 112B, 112C and113A, 113B, 113C, respectively, thermal energy (e.g., heat) istransferred from thermal transfer fluid across inner tubular members 140into production fluids. As a result, during operations with exchanger300, the temperature of production fluid increases to a maximum atoutlet 102 while the temperature of thermal transfer fluid decreases toa minimum when it enters recirculation line 330. Thus, through use ofexchanger 300, relatively cool production fluids are warmed in order toavoid potential problems associated with such cool production fluidssuch as, for example hydrate formation.

Since exchanger 300 is designed to warm production fluids as previouslydescribed, in some embodiments, recirculation line 230 is not includedwhile still complying with the principles disclosed herein. In addition,in at least some embodiments, an additional valve assembly (not shown)is disposed along outlet line 220 which is actuatable to selectivelyrestrict the flow of thermal transfer fluids along line 220 into fluidflow path 113A in row 110A. Thus, in these embodiments, both the valveassembly along line 220 and valve assembly 320 may be actuated toprevent fluid flow of thermal transfer fluids both into and out of rows110A, 110B, 110C of heat exchanger units 120. Without being limited tothis or any other theory, such a flow arrangement may be used to providea substantially stagnate volume of thermal transfer fluid around tubes140 within exchanger units 120, which thus provides an additionalinsulative barrier to heat transfer between production fluids and theocean environment. In at least some of these embodiments, additionalheating devices, similar to those described above to be included withinpumping unit 310 may be disposed throughout units 120 within exchanger300 to thereby maintain a desired temperature of the stagnant thermaltransfer fluids trapped between the closed valve assembly on line 220(not shown) and valve assembly 320.

In addition, in at least some embodiments, an additional flushing lineis included for flushing or removing spent thermal transfer fluid. Forexample, referring still to FIG. 12, exchanger 300 includes a flush line400 extending from valve assembly 320 to a valve assembly disposed alongjumper line 17 downstream of exchanger 300 (see FIG. 1). In thisembodiment, valve assembly 320 is additionally actuatable (i.e.,additional to the positions described above) to a position where thermaltransfer fluid is allowed to flow from flow path 113C in row 110C toflush line 400 and is restricted from flowing into either of the lines230, 330. During normal operations, when thermal transfer fluid isrouted through either line 230 or line 330, valve assembly 320 isactuated to prevent thermal transfer fluid from entering flush line 400.Additional isolation valves 420 are disposed on each of therecirculation lines 230, 330 and flush line 420 to allow for further andfiner control of the routing of fluids during operations. Also, in thisembodiment flush line 400 includes a one way check valve 430 that isconfigured to only allow flow along line 400 from valve assembly 320toward valve assembly 410. Further, in this embodiment, injection points(or ports) 415 (e.g., ROV injection points) are included along bothrecirculation lines 230, 330, and each provides an access point for theinjection or withdrawal of fluids from the respective line 230, 330 byan ROV or other suitable device (e.g., umbilical) during operations.

During operations, when it becomes desirable to remove and/or replacethe thermal transfer fluid flowing through exchanger 300, valve assembly320 is actuated to prevent thermal transfer fluid from entering eitherof the recirculation lines 230, 330, but allow fluid flow into flushline 400 as previously described. Once thermal transfer fluid flows intoline 400 it is routed toward valve assembly 410 and into line 17, whereit can either be sent to the surface (e.g., through riser assembly 20)or routed subsea to some other suitable location or collection point(e.g., subsea tank). During these operations, pumping unit 310 continuesto run in order to provide the necessary pressure differential to causepositive flow of thermal transfer fluid jumper 17 through line 400.After all or substantially all of the spent thermal transfer fluid isflushed from exchanger 300 through line 400, fresh thermal transferfluid is injected (e.g., by an ROV, pipeline, umbilical, etc.) withinline 230 and/or line 330 at injection points 415, thereby refillingexchanger 300. In addition, once all spent thermal transfer fluids areflushed from exchanger 300 through line 400, valve assembly 320 isactuated to once again restrict flow through line 400 while allowingflow through recirculation line 230 and/or line 330 as previouslydescribed above.

In addition to the flushing operations described above, it should alsobe appreciated that flush line 400 may also be utilized to flowdisplaced thermal transfer fluid from exchanger 300 to line 17 whenother fluids and/or additives are being injected at injection points 415in order to prevent an over pressurization of exchanger 300. Injectedadditives and/or fluid may include, for example, corrosion inhibitors,biocides, plasticizers, hydrate inhibitors, or some combination thereof.Further, it should also be appreciated that in some embodimentsinjection points 415 may also be used to induce an initialpressurization of the thermal transfer fluid within exchanger 300 tofacilitate future sampling of the thermal transfer fluid for conditionmonitoring of the fluid properties.

In the manner described, embodiments of subsea heat exchangers describedherein (e.g., exchangers 100, 300) can be used to cool production fluidsthat are produced with temperatures above the operating temperaturerange of downstream equipment in the production system (e.g., system 10)or heat production fluids that are produced with temperatures below theoperating temperature range of downstream equipment in the productionsystem (e.g., system 10). In addition, the modular design of embodimentsof subsea heat exchangers described herein (e.g., exchangers 100, 300)enables the heat exchangers to be tailored for to the desired thermaltransfer performance by adding or removing modular heat exchanger units(e.g., units 120) to closely match the specifications and/or needs ofthe associated production system (e.g., system 10).

While certain exemplary embodiments have been shown and described,modifications thereof can be made by one of ordinary skill in the artwithout departing from the scope of teachings herein. For example insome embodiments, thermal insulation may be disposed about each of theheat exchanger units 120 and all associated piping within exchangers100, 300 in order to reduce the thermal influence of the oceanenvironment during operations. However, it should be appreciated that inat least some of these embodiments the sections and 254 and curves 252within radiator 250 may be left uncovered to enhance heat transferbetween the thermal fluid flowing therethrough and the oceanenvironment. As another example, while embodiments disclosed herein haveshown a heat exchanger (e.g., exchanger 100, 300) receiving productionfrom a single wellbore (e.g., wellbore 14), it should be appreciatedthat in other embodiments, exchanger 100 and/or 300 may receiveproduction fluids from more than one such wellbores while stillcomplying with the principles disclosed herein. In addition, in at leastsome embodiments, one or more of the tubes 140 may be replaced with aplurality of tubes extending parallel to one another (e.g., parallel toaxis 125) rather than a single length of pipe. Further, some embodimentsof exchangers 100, 300 may also include a hot stab panel or othersuitable access point for an ROV or similar device, to enable injectionof fluids (e.g., warm fluids, hydrate inhibitors, cleaning solutions,etc.) into any portion of exchanger 100, 300. Still further, in someembodiments, pumping units 210, 310 may be utilized to providepressurized thermal transfer fluid to more than one exchanger 100, 300,respectively, while still complying with the principles disclosedherein. Also, in at least some embodiments, pumping units 210, 310 mayinclude one or more filter modules to capture particulate matter that isdisposed within thermal transfer fluid, thereby reducing the likelihoodof clogging or other failures caused by such suspended particulatematter. Moreover, while the subsea heat exchangers shown and describedherein (e.g., exchangers 100, 300) include a plurality of heat exchangerunits 120 that are arranged such that fluids (e.g., production fluid,thermal transfer fluid) flows generally in an S-shaped pattern (e.g.,see FIGS. 8 and 10), it should be appreciated that other arrangementsmay be used. For example, in some embodiments, heat exchanger units 120are stacked vertically upon one another rather than being spread alongsea floor 7 as shown. Thus, the embodiments described herein areexemplary only and are not limiting. While embodiments disclosed hereinhave included flanges (e.g., flanges 150, 160) for connecting heatexchangers 120 to one another (e.g., through tubing members 222 andtransfer pipes 104, 106, bridging assemblies 170, etc.) otherembodiments may replace one or more of the flanges 150, 160 with weldedconnections in order to eliminate the need for a hydrocarbon containingseal (e.g., seal assemblies 151). In addition, other suitable,non-flanged connections may be utilized such as a clamp connector(hydraulic or otherwise) and the like. Also, it should be appreciatedthat other valve assemblies may be utilized within the exchangers 100,300 in addition to those specifically shown and described above, whilestill complying with the principles disclosed herein. For example, insome embodiments, additional valves/valve assemblies are attached inand/or around the pumping module 210, as well as along lines 220, 230,330, etc.

In addition, many other variations and modifications of the systems,apparatus, and processes described herein are possible and are withinthe scope of this disclosure. Accordingly, the scope of protection isnot limited to the embodiments described herein, but is only limited bythe claims that follow, the scope of which shall include all equivalentsof the subject matter of the claims. Unless expressly stated otherwise,the steps in a method claim may be performed in any order. Therecitation of identifiers such as (a), (b), (c) or (1), (2), (3) beforesteps in a method claim are not intended to and do not specify aparticular order to the steps, but rather are used to simplifysubsequent reference to such steps.

What is claimed is:
 1. A subsea heat exchanger, comprising: a productionfluid inlet; a production fluid outlet; a first heat exchanger unit anda second heat exchanger unit each coupled to the inlet and the outlet,wherein each heat exchanger unit has a central axis, a first end, and asecond end opposite the first end, and wherein each heat exchanger unitcomprises: an outer tubular member extending axially from the first endto the second end of the heat exchanger unit; an inner tubular memberdisposed within the outer tubular member, wherein the inner tubularmember extends axially from the first end to the second of the heatexchanger unit; an annulus radially disposed between the inner tubularmember and the outer tubular member; and a bridging assembly coupled tothe second end of the first heat exchanger unit and the second end ofthe second heat exchanger unit, wherein the bridging assembly includes:a connector having a central connector axis, a first end coupled to thesecond end of the first heat exchanger unit, a second end coupled to thesecond end of the second heat exchanger unit, and a throughbore incommunication with the inner tubular member of the first heat exchangerunit and the inner tubular member of the second heat exchanger unit; anda tubular stab having a central stab axis oriented parallel to andradially spaced from the connector axis, wherein the tubular stabfluidly couples the annulus of the first heat exchanger unit to theannulus of the second heat exchanger unit.
 2. The subsea heat exchangerof claim 1, wherein the inner tubular member of each heat exchanger unitincludes a flange disposed at the second end of the heat exchanger unit;wherein each flange includes a port extending parallel to and radiallyspaced from the central axis of the corresponding heat exchanger unit;and wherein a first end of the tubular stab is received within the portin the flange of the inner tubular member of the first heat exchangerunit, and a second end of the tubular stab is received within a port inthe flange of the inner tubular member of the second heat exchangerunit.
 3. The subsea heat exchanger of claim 2, wherein at least one ofthe first end and the second end of the tubular stab is freely slidablewithin the corresponding port.
 4. The subsea heat exchanger of claim 2,wherein each flange further includes a radially outer annular surfacethat slidingly and sealingly engages a radially inner surface of thecorresponding outer tubular member.
 5. The subsea heat exchanger ofclaim 1, wherein the bridging assembly includes a plurality of tubularstabs uniformly circumferentially spaced about the central connector,wherein each tubular stab has a central stab axis oriented parallel toand radially spaced from the connector axis, wherein the plurality oftubular stabs fluidly couple the annulus of the first heat exchangerunit to the annulus of the second heat exchanger unit.
 6. The subseaheat exchanger of claim 1, further comprising a closed thermalprocessing loop in fluid communication with the annulus of each of thefirst heat exchanger unit and the second heat exchanger unit, whereinthe thermal processing loop is configured to circulate a thermaltransfer fluid through the annulus of the first heat exchanger unit, thetubular stab, and the annulus of the second heat exchanger unit.
 7. Thesubsea heat exchanger of claim 6, wherein the thermal processing loopfurther includes a radiator configured to cool the thermal transferfluid.
 8. The subsea heat exchanger of claim 1, further comprising: athird heat exchanger unit coupled to the inlet and the outlet, whereinthe third heat exchanger unit includes a central axis, a first end, anda second end opposite the first end, and wherein the third heatexchanger unit further comprises: an outer tubular member extendingaxially from the first end to the second end of the third heat exchangerunit; an inner tubular member disposed within the outer tubular member,wherein the inner tubular member extends axially from the first end tothe second of the third heat exchanger unit; and an annulus radiallydisposed between the inner tubular member and the outer tubular member;a first stiffening plate secured to the first end of the first heatexchanger unit and the first end of the third heat exchanger unit; and asecond stiffening plate secured to the second end of the first heatexchanger unit and the second end of the third heat exchanger unit;wherein the first stiffening plate and the second stiffening plate areconfigured to support all of the weight of the first heat exchanger unitand the third heat exchanger unit.
 9. The subsea heat exchanger of claim8, wherein the central axis of the first heat exchanger unit is parallelto and radially spaced from the central axis of the third heat exchangerunit.
 10. The subsea heat exchanger of claim 1, where each heatexchanger unit further comprises a plurality of baffles disposed withinthe annulus, wherein the baffles are configured to induce a sinusoidalflow path for thermal transfer fluid.
 11. The subsea heat exchanger ofclaim 10, where each baffle includes a radially outer curved surfacethat sealingly engages with a radially inner surface of thecorresponding outer tubular member.
 12. An offshore production systemfor producing hydrocarbon fluids from a subterranean well, the systemcomprising: a production tree disposed at the sea floor, wherein theproduction tree includes a plurality of valves configured to control aflow of hydrocarbon fluids from the subterranean well; a riser assemblyfluidly coupled to the production tree and configured to flow thehydrocarbon fluids to a vessel disposed at the sea surface; a heatexchanger disposed on the sea floor and including: an inlet configuredto receive the hydrocarbon fluids from the production tree; an outletconfigured to supply the hydrocarbon fluids to the riser assembly; aplurality of heat exchanger units coupled to the inlet and the outlet,wherein each heat exchanger unit has a central axis, a first end, and asecond end opposite the first end, and wherein each heat exchanger unitcomprises: an outer tubular member extending axially from the first endto the second end of the heat exchanger unit; an inner tubular memberdisposed within the outer tubular member, wherein the inner tubularmember extends axially from the first end to the second of the heatexchanger unit, and wherein the inner tubular member of each heatexchanger unit is in fluid communication with the inlet and the outlet;and an annulus radially disposed between the inner tubular member andthe outer tubular member; and a closed thermal processing loop in fluidcommunication with the annulus of each of the heat exchanger units,wherein the thermal processing loop is configured to circulate a thermalprocessing fluid through the annuli of the plurality of heat exchangerunits.
 13. The offshore production system of claim 12, wherein the innertubular member of each heat exchanger unit includes a pair of flanges,with one flange is disposed at each of the first end and the second endof the corresponding heat exchanger unit; wherein each flange includes aport extending parallel to and radially spaced from the central axis ofthe corresponding heat exchanger unit.
 14. The offshore productionsystem of claim 13, wherein each flange further includes a radiallyouter annular surface that slidingly and sealingly engages a radiallyinner surface of the corresponding outer tubular member.
 15. Theoffshore production system of claim 13, wherein the heat exchangerfurther includes a bridging assembly coupled to the second end of afirst of the plurality of heat exchanger units and the second end of asecond of the plurality of heat exchanger units, wherein the bridgingassembly includes: a connector having a central connector axis, a firstend coupled to the flange at the second end of the first heat exchangerunit, a second end coupled to the flange at the second end of the secondheat exchanger unit, and a throughbore in communication with the innertubular member of the first heat exchanger unit and the inner tubularmember of the second heat exchanger unit; and a tubular stab having afirst end disposed within the port in the flange at the second end ofthe first heat exchanger unit and a second end disposed within the portin the flange at the second end of the second heat exchanger unit. 16.The offshore production system of claim 15, wherein at least one of thefirst end and the second end of the tubular stab member is freelyslidable within the corresponding port.
 17. The offshore productionsystem of claim 15, wherein the heat exchanger further includes: atransfer pipe in fluid communication with the inner tubular member ofthe first heat exchanger unit and the inner tubular member of a third ofthe plurality of heat exchanger units; and a transfer tube in fluidcommunication with the annulus of the first heat exchanger member andthe annulus of the third heat exchanger member; wherein the transfertube has a first end disposed within the port in the flange at the firstend of the first heat exchanger unit and a second end disposed withinthe port in the flange at the first end of the third heat exchangerunit; and wherein central axis of the third heat exchanger unit isparallel to and radially spaced from the central axis of the first heatexchanger unit.
 18. The offshore production system of claim 17, whereinthe heat exchanger further includes: a first stiffening plate secured tothe first end of the first heat exchanger unit and the first end of thethird heat exchanger unit; and a second stiffening plate secured to thesecond end of the first heat exchanger unit and the second end of thethird heat exchanger unit; wherein the first stiffening plate and thesecond stiffening plate are configured to support all of the weight ofthe first heat exchanger unit and the third heat exchanger unit
 19. Theoffshore production system of claim 12, wherein each of the plurality ofheat exchanger units further includes a plurality of baffles disposedwithin the annulus, wherein the baffles are configured to induce asinusoidal flow path for thermal transfer fluid.
 20. The offshoreproduction system of claim 19, wherein each baffle includes a radiallyouter surfaced surface that sealingly engages with a radially innersurface of the corresponding outer tubular member.
 21. A method forcooling hydrocarbon fluids produced from an offshore subterranean well,the method comprising: (a) producing hydrocarbon fluids from aproduction tree disposed at the sea floor to an inlet; (b) flowing thehydrocarbon fluids from the inlet through an inner tubular member of afirst heat exchanger unit; (c) flowing the hydrocarbon fluids through aconnector of a bridging assembly into an inner tubular member of asecond heat exchanger unit; (d) flowing a thermal transfer fluid throughan annulus of the first heat exchanger unit; and (e) flowing the thermaltransfer fluid through a tubular stab of the bridging assembly into anannulus of the second heat exchanger unit.
 22. The method of claim 21,further comprising cooling the thermal transfer fluid after (d) and (e).23. The method of claim 22, wherein cooling the thermal transfer fluidcomprises flowing the thermal transfer fluid through a radiator.
 24. Themethod of claim 21, further comprising heating the thermal transferfluid before (d) and (e).
 25. The method of claim 21, furthercomprising: (f) flowing the hydrocarbon fluids through a transfer pipeinto an inner tubular member of a third heat exchanger unit after (c);and (g) flowing the thermal transfer fluid through a transfer tube intoan annulus of the third heat exchanger unit after (e).
 26. The method ofclaim 21, further comprising recirculating the thermal transfer fluid tothe annulus of the first heat exchanger unit after (d) and (e).